Backfiring and missing 1972 351C-4V in storage


New Member
Mar 31, 2005
I have a 72,Mach 1 with a 351C,(Q-code),C-6,with the high compression big valve/port quench heads that`s been off the road in storage at the old cottage in a garage since 1990.It`s still fairly solid,but needs a restoration.
(I`ve somehow managed to keep my first car around)
Over the years,(every),I would start it up occasionally during spring,summer and fall months and occasionally take it out for a spin on the back roads.

It`s run fairly well up to about two years ago when it started acting up.
It started gradually running rougher,so I figured it`s probably a fuel system problem due to the age of the parts.
I rebuilt the 750 vac-secondary Holley carb,(actually it was pretty clean inside,no excessive gum/varnish) cleaned it well with carb cleaner and blew all the passageways out with compressed air,installed a brand new mechanical fuel pump,fuel filter and disconnected the fuel line from the tank and ran the engine off a large gas can.(over the years in the fall I would run the tank down low and add fresh gas in the spring,and started using fuel stablizer three years ago).

Still ran like crap after this.It would start up fast and then start missing and backfiring through the exhaust,becoming gradually worse,so I parked it and last summer tried a 600 vac-sec Holley carb that I had used previously on the car.No improvement.Car starts up fast then turns into a missing sonic BOOM machine out the exhaust.Last summer my brother was moving the car,gave it a little too much gas and literally blew up both mufflers,(I wasn`t there at the time they blew).After both mufflers blew up, the car was not restarted until this spring.We cut off the blown mufflers leaving the exhaust open two weeks ago.
We thought it may have been a plugged exhaust,but no,it`s still backfiring and missing like crazy and dies if you take your foot off the gas.

I checked for major vacuum leaks.Nothing.
I took out the spark plugs,(Motorcraft).They were on the black side and a little cruddy,but didn`t have a strong gasoline smell on any of them.I cleaned them up and put them back in,started it up and no improvement,whatsoever.

The engine was rebuilt in the late 1980`s and doesn`t really have a lot of miles on it.
The engine has the following,600 Holley vac-sec now,750 Holley vac-sec before,Edelbrock 351-4V dual plane intake,a mild Crower "power compu-pro cam",210int-218exh @ .050",Hedmann headers.

The ignition consists of an Accel electronic distributor,41000E and S series,(not sure which one as the label is long gone),with mechanical and vacuum advance,(which appear to be working fine) with an electronic module on the side of the distributor.
Accel super stock coil.
Accel cap and rotor in good condition and higher quality Accel plug wires.
I haven`t had time to check out the ignition system more closely,but like I said the car fires right up.
The timing is set to factory spec.
If not an ignition problem,I`m possibly thinking of internal issues like sticking valves and lifters.
BTW last summer I did pull the valve covers and manually moved each valve up and down with a pry bar while spraying penetrating fluid around the stem and spraying down at the lifters.It didn`t help at all.
I`m pretty sure the timing chain I used when rebuilding was a good quality roller chain and with the relatively low mileage,I can`t see it really jumping a tooth or two.
Sorry for the long post,but I wanted to mention everything I could think of.
Any advice/suggestions.

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New Member
Mar 31, 2005
69 Capecod said:
check the oil there is a shelf life to it. Have you changed it?

I stored it with new 15w50 semi synthetic in 1990,didn`t bother changing it till this year with Mobil 1 15w50.
It always looked clean on the dip stick and when I had the valve covers off last year the top of the heads were clean/like sludge at all.


Founding Member
Mar 27, 2002
Oak Ridge, NJ
I'm hoping someone figures this out, because I'm having the EXACT same problem, with a similar setup and the same kind of long term storage.

I was thinking a valve spring problem from the car sitting so long.

Anyone think this is a possibility?


Jan 3, 2003
While it is running, rev it up and spray some carb cleaner in the carb. Also, spray some carb cleaner around the bottom of the intake. You may have a dried up gasket and it is causing a vacuum leak. If it smooths out, then replace the intake gasket. Also, check all your vacuum lines for rotting. I would replace all of them anyway.